Rasch analysis of the Norwegian version of the Arthritis Self-Efficacy Scale (ASES).

Objectives: To apply the Rasch model to the Norwegian version of the Arthritis Self-Efficacy Scale (ASES).Method: The ASES was included in a self-administered questionnaire sent to 224 patients attending one of three rehabilitation centres for rheumatic diseases in Norway in 2009. The fit of the ASES to the Rasch model was assessed together with hypothesized associations with the Modified Health Assessment Questionnaire (MHAQ), the 36-item Short Form Health Survey (SF-36), the numerical rating scale (NRS) for pain, and NRS fatigue.Results: A total of 145 (64.7%) patients responded to the questionnaire. The two scales of other symptoms and pain showed good fit to the Rasch model with no evidence for differential item functioning (DIF) according to eight sociodemographic and disease-related variables. The Person Separation Index (PSI), which is equivalent to Cronbach's alpha, ranged from 0.74 to 0.78. Correlations with scores for other instruments were as hypothesized: ASES pain had the highest correlations with SF-36 pain and NRS pain and ASES other symptoms had the highest correlations with other aspects of the SF-36 and NRS fatigue.Conclusions: The ASES had good fit to the Rasch model and correlations with other instrument scores that followed hypotheses, lending further support to the application of the instrument in patients with rheumatic diseases.